A series of experiments is proposed for investigating the nature of the basic capacities underlying speech perception in infants and the subsequent development of these capacities in the process of acquiring a native language. Data concerning the infant's discrimination and categorization of speech during the first year of life will be collected using various sucking, visual preference and operant headturning procedures. The information gained from these investigations with infants will provide not only an index of the nature of the basic capacities underlying speech perception, but also an indication of the way these capacities are affected during the infant's first attempts to acquire the sound structure of the language spoken in his or her native environment. Research with older children and adults using categorization measures serves to delineate further the way in which basic speech perception capacities develop as a result of acquiring and using a particular language. Special attention is given to the way in which the organization governing the phonological (i.e. sound) structure of the language affects speech perception. The proposed research, then, aims both to identify the basic capacities underlying speech perception and to define the set of relevant experiences which shape the way in which adults process speech signals. This research has practical relevance for those studying communications disorders. Information about the way in which speech perception develops provides a baseline for assessing abnormalities in processing speech. Early identification of the development of such abnormalities is essential in formulating effective treatment programs.